Ridged waveguide slot antenna



June 15, 1965 J. H. PROVENCHER 3,189,908

RIDGED WAVEGUIDE SLOT ANTENNA Filed Jan. 22, 1962 2 Sheets-Sheet 1 FIG. I

(PRIOR ART) INVENTOR. JOSEPH PRO VE/VCHER June 15,1965 J. H. PROVENCHER 3,189,903

RIDGED WAVEGUIDE SLOT ANTENNA v 2 Sheets-Sheet 2 Filed Jan. 22, 1962 INVENTOR. JOSEPH H. PHOVENCHER 6? A. ATTOR 5X5 United States Patent 3,189,968 RIDGED WAVEGUlDE SLOT ANTENNA Joseph H. Provencher, San Diego, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Jan. 22, 1962, Ser. No. 167,983 19 Claims. (Cl. 3 t3- 771) (Granted under Title 35, US. Code {1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to antennas and more particularly, to a slot antenna and specifically, to a slotted ridged waveguide.

One method of radiating or receiving electromagnetic energy is through the use of a series of discrete slots in a waveguide. These slots are usually resonant for a given frequency and are normally spaced approximately one-half guide wavelength apart and additionally, are alternated on either side of the waveguide center line. The radiation pattern of such an array of slots is then controlled by a given amplitude distribution along the length of the array. The contribution from each slot is controlled by the displacement of the slot off the waveguide center line.

The staggering of the slots about the center line causes undesirable radiation outside of the principal planes and this radiation becomes more pronounced as the off-center displacement is increased. In that the slots are approximately one-half wavelength long, their length is frequency dependent and this limits the operation of such an anten- 11a over a relatively narrow frequency band. Such frequency dependency results in pattern deterioration and large impedance changes as the frequency is moved away from a resonant frequency. In addition, precise machining of slot length and slot spacing is required thereby causing the entire antenna to be highly resonant.

In addition, the width of a slotted waveguide imposes somewhat of a restriction on its use as an element in a parallel array, especially when spacings on the order of a half wavelength are required. This width restriction, however, can be reduced by loading the waveguide, either with a dielectric or through the use of a ridge. In either case, attenuation will result but may be minimized by proper choice of the loading material or ridge dimensions.

An object of the present invention is to provide a slotted waveguide antenna wherein the length of the slot is not directly related to the operating frequency and will not be required to be resonant at that frequency.

A further object of the invention is to provide a slotted waveguide antenna wherein precise machining of slot length and slot spacing is not required.

An additional object of the invention is to provide an antenna which is capable of scanning both in azimuth and elevation.

An additional object of the invention is to provide a slotted waveguide antenna capable of radiating large amounts of power from a small physical space, i.e., couple out the most power with the smallest number of slots.

An additional object of the invention is to provide a slotted waveguide antenna which has a stable radiation pattern and an acceptable impedance level over a relatively broad frequency band.

Another object of the invention is to provide a slotted waveguide antenna capable of radiating a large amount of power from a slot relatively close to the center line of the waveguide.

A further object of the invention is to provide a slotted waveguide antenna which has a stable pattern and stable impedance as the frequency is moved away from the reso nant frequency.

An additional object of the invention is to provide a slotted waveguide antenna wherein undesirable radiation outside the principal planes is reduced.

A further object of the invention is to provide a slotted waveguide antenna wherein power extracted from the slot is relatively independent of the center line displacement of the slots, hence, requiring less stringent tolerances in manufacture.

In order to better understand the embodiments of the invention reference is made to the following drawings wherein:

FIG. 1 illustrates the prior art slotted waveguide;

FIG. 2 illustrates one embodiment of the present invention;

FIG. 3 illustrates another embodiment of the present invention showing slots staggered on either side of the center line;

FIG. 4 illustrates an embodiment of the present invention incorporating the height finding feature; and

FIG. 5 illustrates another embodiment of the invention having the radiating slots on the same side of the center line waveguide.

As shown in FIG. 1 the prior art slotted waveguide antennas comprised a rectangular waveguide 10 having short sides 12 and 14 and long sides 16 and 18. Mounted internally on one long side as on side 18 is a loading ridge 2% which extends internally of the waveguide. The loading ridge increases the cut-oil? wavelength, and widens the frequency range over which only the dominant mode will propagate. In the wall 16 opposite to wall 18 upon which the loading ridge 20 is mounted, a radiating slot 22 is provided.

As is well-known, a hole or joint or slot in a waveguide wall introduces the possibility that energy will leak from the guide to outer space. When this happens, fields inside the guide are affected, thereby introducing an irregularity with resulting reflection. Thus, when magnetic flux breaks through radiating slot 22 the associated interference with the flow of currents in the wall produces voltage across the slot 22 that gives rise to an electric field that will extend outside the guide.

In the embodiment of the invention illustrated in FIG. 2, a rectangular waveguide 24 is provided having short sides 26, 28 and long sides 30, 32. Mounted on one long wall of the waveguide 24- as on wall 32 is a loading ridge 34 which extends internally into the hollow waveguide 24. Also, cut into long wall 32 is a longitudinally extending radiating slot 36. As shown, the slot is off the center line of the waveguide and lies outside the area on wall 32 bounded by the loading ridge 34. This is done so that the area inside loading ridge 34 is essentially a dead area.

In the embodiment of FIG. 2, the length of the slot is not directly related to the operating frequency and will not be resonant at that frequency. Generally, for the shunt slot cut in a ridged Wave-guide such as shown in FIG. 1 maximum power is coupled out when the slot is of resonant length, which is in the vicinity of 0.5 free space wavelength. However, in the present invention a slot length which gives the maximum coupled :power is in the vicinity of 0.6 free space wavelength. In addition, the power level of the embodiment shown in FIG. 2 is relatively higher than that of the resonant slot cut in the wall of a waveguide opposite the ridge, such as shown in the prior art of FIG. 1. For short arrays, where large amounts of power need to be removed by a few slots, there would be a decided advantage in this type of configuration. Under other conditions, i.e., for long arrays, it would be possible to remove the power with fewer slots or a greater percentage of the power in the same length, by utilizing an array incorporating the features of the invention set forth in FIG. 2 in conjunction with a number of conventional longitudinal shunt slots. Further, it is noted that similar a it results are obtained at 100 megacycles on either side of the center frequency of 3,000 megacycles and this would indicate that the slot of FIG. 2 behaves in a similar manner for given bandwidths whereas the slot of FIG. 1, in that it is resonant at a single frequency, does not give the relatively broad bandwidth.

FIG. 3 illustrates an embodiment of the invention utilizing radiating slots positioned on alternate sides of the Waveguide center line with a shorting plate properly positioned on each end. An array of this type exhibits a stable radiation pattern over a relatively broad frequency band. The array comprises a waveguide 40 having short sides 42, 44; long sides 46, 48 and a loading ridge 50 positioned on one of the long sides such as side 48. Positioned on each end of the waveguide 40 is a shorting plate 52. Spaced on alternate sides of the center line and outside of the area bounded by the ridge 50 are radiating slots 54 which are spaced apart in the long direction of the waveguide in integral multiples of one-half guide wavelength. The length of the slots is approximately 0.6 free space Wavelength of the middle operating frequency.

FIG. 4 illustrates another embodiment of the invention wherein a dual frequency feature is added to the antenna. In FIG. 4, rectangular waveguide 56 is provided having short sides 53, 60 and long sides 62, 64. Positioned on one long side of the waveguide 56, say on side 64, is a loading ridge 66. A radiating slot 68 is provided adjacent the area bounded by the loading ridge 66 in wall 64 for the lower operating frequency band. Another slot '70 is provided in the wall 64 within the area bounded by the loading ridge 66 and provides a means of obtaining an independent beam for a higher operating frequency band. Essentially then, there is a waveguide positioned Within a Waveguide, i.e., ridge 66 acts as one waveguide while the main Waveguide 56 also performs its function. This technique can be extended to three or more waveguides, each with its independent slot array for the appropriate frequency band.

The embodiment of FIG. 5 exhibits a stable radiation pattern with good impedance characteristics over a relatively broad frequency range. This embodiment comprises a section of rectangular waveguide 72 again having short sides 74, 76 and long sides 78, 80. Both ends are closed by shorting plates 82 and 84. Mounted on one of the long sides as on side 78 is a loading ridge 86. Cut into the same wall that the loading ridge is mounted on are two radiating slots 88 which are approximately 0.6 free spaced wavelengths long and positioned approximately one-half guide wavelength apart in the axial direction of the waveguide 72. The feed to this 2-slot element will enter the Waveguide at the midpoint between them, as does probe 90.

In the operation of the embodiments of the invention the antennas may be center fed or end fed, short circuited or loaded at the ends. Through the use of the slot longer than one-half wavelength at the operating frequenc the band-width restrictions of the resonant slot are avoided.

Experimental slot radiating data, taken by well-known methods, of the conventional slot ridged waveguides, contrasted with the adjacent slot ridged Waveguides has been plotted and is available. These curves show a different behavior for both types of slot. For any given displacement off the center line of the waveguide, the conventional slot radiates at a lower power level than the adjacent slot, and has a shorter length than the adjacent slot. The experimental frequency chosen was one in which the length of the conventional slot was near resonance. For this frequency and with a given waveguide configuration, the maximum radiation for the conventional slot was about 50%, While that of the adjacent slot was about 73% of input power. Further, the displacement of the conventional sl-ot was 0.650 inch off the center line, while the displacement of the adjacent slot was 0.360 inch off the center line of the waveguide. Thus, the use of the adjacent slot would allow a more compact array than the use of the conventional slot. In general, the relative power level of the adjacent slot, for any given displacement ofi center line is higher than that of the conventional slot.

In addition, through the use of the adjacent slot a stable radiation pattern and an acceptable impedance level over a relatively broad frequency band is possible. If shaping of the radiated beam is desired or if it is desired to reduce back radiation, chokes or wings may be placed on the sides of the slot and the entire configuration may be placed in a conducting sheet.

In a summary, when a waveguide is excited in a suitable manner, electromagnetic waves travel down the waveguide in uniform pattern. In the conventional waveguide, if a longitudinal slot is cut in the wall of the long side, transverse currents will be intercepted and an electric field will appear across the slot. Thus, the slot is adapted for the interchange of energy between the waveguide and the surrounding space. portional to the displacement of the slot oif the waveguide center line. By placing additional slots staggered about the center line, spaced one-half guide wavelength apart along the waveguide and having a length about one-half guide wavelength, the fields from the individual slots can be made to radiate approximately in phase so that the energy is directed in a concentrated beam in a direction roughly perpendicular to the face of the waveguide.

'In the adjacent slot case the OE center line displacement can be held constant and the amplitude distribution can be controlled by the length of the slot i.e., for the slot cut in the wall containing the loading ridge. The two-slot configuration such that shown in FIG. 5 can be used as an individual element in a planar array, thereby eliminating the need to stagger the slots to maintain an inphase array. Additionally, an array such as that shown in FIG. 3 can be made which exhibits a stable radiation pattern for a relatively broad frequency band. In this array the metal plate 52 is placed about a quarter guide wavelength beyond the last slot 54 at the end of waveguide 40.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An antenna comprising; a rectangular waveguide; a loading ridge mounted on and projecting internally from a wall of said rectangular waveguide; at least one radiating slot located adjacent to the ridge in the same wall that the loading ridge is mounted on.

2. An antenna as set forth in claim 1 wherein said at least one radiating slot is parallel to the axis of propagation of energy in said rectangular waveguide.

3. An antenna as set forth in claim 1 wherein at least one slot has a physical length greater than one-half free space wavelength at the operating frequency.

4. An antenna as set forth in claim 1 wherein the physical length of said radiating slot is greater than onehalf free space wavelength and is substantially equal to one-half guide wavelength at the operating frequency.

5. An antenna comprising; a rectangular waveguide; a loading ridge located internally of said rectangular width waveguide and on one wall of said rectangular waveguide; a slot located in the wall that the rectangular loading ridge is mounted on; said slot being parallel to the axis of propagation of energy in the waveguide.

'6. An antenna as set forth in claim 5 wherein said loading ridge is located on a Wall of said rectangular waveguide having a long dimension as compared to the two walls of said rectangular waveguide having a short dimension.

7. A broadband frequency antenna comprising; a rectangular waveguide having two walls with a long dimension and two walls with a short dimension; a loading ridge mounted internally in said waveguide and on one of said walls of said waveguide; loading slots located in said wall The magnitude of the nadiation is proof said rectangular Waveguide having said loading ridge mounted thereon; and said rectangular waveguide adapted to be center fed.

8. A broadband frequency antenna as set forth in claim '7 wherein said radiating slots have a physical length of substantially one-half guide wavelength of the center operating frequency.

9. A broadband frequency antenna as set forth in claim 8 wherein said radiating slots are spaced one-half guide Wavelength apart at the operating frequency.

'10. A broadband frequency antenna as set forth in claim 7 wherein said radiating slots are positioned on the same side of the waveguide center line.

PM. A broadband frequency antenna as set forth in claim 7 wherein the number of radiating slots are two so that said antenna exhibits a stable radiation pattern and good impedance characteristics over a relatively broad M frequency band.

1 2. A broadband frequency antenna as set forth in claim 8 wherein said radiating slots are spaced apart integral multiples of substantially one-half guide wavelength at the operating frequency.

'13. A broadband frequency antenna as set forth in claim '7 wherein said radiating slots are positioned on alternate sides of the waveguide center line.

14. A broadband frequency antenna as set forth in claim 13 and further comprising; a shorting plate positioned on either end of said rectangular waveguide so that said broad-band frequency antenna exhibits a stable radiation pattern over a relatively broad frequency band.

15. An antenna comprising; a rectangular waveguide; a loading ridge mounted on and extending internally of one long .wall; at least one radiating slot located adjacent to the loading ridge in the same wall that the loading ridge is mounted on for radiating energy at a first frequency;

at least one other slot located in the same wall that the ioad-ing ridge is mounted on and within the area bounded by said loading ridge for radiating energy at another frequency so that said antenna may scan in a horizontal and vertical direction simultaneously.

:16. An antenna as set forth in claim 15 wherein said at least one radiating slot located adjacent to said ridge and said at least one other slot located within the area bounded by said ridge are parallel to the axis of propagation of energy in said waveguide.

17. An antenna as set forth in claim 16 wherein said at least one radiating slot located adjacent to the loading ridge has a physical length greater than one-half free space Wavelength at the operating frequency.

18. An antenna as set forth in claim 16 wherein said loading ridge constitutes a separate waveguide and Wherein said at least one other radiating slot located within the area bounded by said loading ridge has a physical length which is greater than one-half the free space Wavelength of said another fnequency.

it An antenna as set forth in claim 18 wherein said at least one other radiating slot length is substantially one half guide wavelength of said another frequency coupled to said loading ridge.

References Cited by the Examiner UNITED STATES PATENTS 10/57 Woodward 343-771 X 6/58 Reed et al 343--771 X OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner, 

1. AN ANTENNA COMPRISING; A RECTANGULAR WAVEGUIDE; A LOADING RIDGE MOUNTED ON AND PROJECTING INTERNALLY FROM A WALL OF SAID RECTANGULAR WAVEGUIDE; AT LEAST ONE RADIATING SLOT LOCATED ADJACENT TO THE RIDGE IN THE SAME WALL THAT THE LOADING RIDGE IS MOUNTED ON. 